Mutants (16 page)

Polydactyly mutations relax control of sonic hedgehog altering the balance of power in favour of ubiquity. But other mutations have exactly the opposite effect and prevent sonic from appearing in the limb-bud at all. The most blatant example of such a mutation is, of course, one that disables the sonic gene itself. Sonic-less mice have, in addition to their many other defects, no paws. This is strikingly reminiscent of a disorder that we have already come across: acheiropody, the disorder of the
aleijadinhos.
Indeed, there is some (disputed) evidence that the acheiropody mutation disables a regulatory element essential to sonic’s presence in the limb.

This catalogue of mutations only hints at the complexities of gene regulation in the embryo. Whether or not a gene is turned on in a given cell depends on what transcription factors are found in that cell’s nucleus, and their presence depends on the presence of yet other transcription factors, and so on. At first glance hierarchies of this sort seem to involve us in an infinite regression in which the burden of producing order is merely placed upon a previous set of entities which must, themselves, be ordered. But this dilemma is more apparent than real. The embryo’s order is created iteratively. Sonic’s precise presence in the ZPA is defined in part by the activity of Hox genes in the trunk mesoderm from which limbs grow. But the geometrical order that these genes give to the limb is crude; sonic’s task is to refine it further. Beyond sonic there are, of course, yet further levels of refinement in which order is created on ever smaller scales, and each of them requires subtle and interminable negotiations, the nature of which we scarcely understand.

This vision of successive layers of negotiation and control may seem unimaginably complex. But in truth it is not complex enough, for it fails to capture one of the most pervasive properties of the embryo’s programme: its non-linearities. I argued that the acheiropody mutation causes a failure of sonic to appear in the limb. And yet I began this chapter by arguing that infants with amputations in the womb, of whatever severity, were due to failures of the apical ectodermal ridge and the fibroblast growth factors they produce. This may seem like a contradiction, but it is only one if we think of the various limbs’ signals as being independent of each other, when in fact they are not. For
one of the most vital roles of sonic hedgehog is to maintain and shape the apical ectodermal ridge and its fibroblast growth factors; and one of the most vital roles of the apical ectodermal ridge is to maintain and shape the production of sonic hedgehog in the zone of polarising activity. There is a reciprocal flow of information as precarious as the flow of batons between two jugglers standing at opposite ends of a stage. Reciprocity of this sort is ubiquitous in the embryo and it alters the way we think about its growth and development. We begin with notions of linear pathways of command and control and simple geometries – and then watch as they unravel. For when, as in the limb, we actually begin to see the outlines of the embryo’s programme, it invariably turns out to resemble a tangle of circuits that loop vertiginously across time and space. Circuits which, in this case, ensure that when we count our fingers and toes we usually come up with twenty.

HANDS, FEET AND ANCESTORS

Around day 32 after conception, when the human limb-bud is already well grown, its amorphous tissue begins to resolve into patterns. Ghostly precursors of bones appear: conglomerations of cells that have migrated together. The technical word for this process is ‘condensation’. It hints at the way in which bones just quietly appear, rather like dew.

The first condensations to form become the bones closest to the body: the humerus in the arm, the femur in the leg. With time, conglomeration sweeps slowly down the limb-bud. The
humerus divides into two new long, thin condensations, each of which will bud off by itself: the radius and the ulna. These condensations, in turn, divide and bud to form an arc of cells from which the twenty-seven bones of the wrist and palm are made. By day 38 after conception, the end of each limb-bud has become flat and broad, rather like a paddle. The paddle then folds into parallel valleys – four on each tip – leaving five islands of condensed cells: the future bones of the fingers and toes.

The shapes of the condensations depend, ultimately, on the reference grid laid down by the signalling systems of the limb. But, as elsewhere in the embryo, this information must be translated into cellular action. Hox genes do this for the head-to-tail axis of the embryo, and they also do it for the limb. As the limb-bud grows, some of the thirty-nine Hox genes appear in intricate overlapping patterns. They seem to be engaged in some combinatorial business analogous to the vertebral Hox code. Infants born with a single defective copy of the Hoxa 13 gene have short big toes and bent little fingers. Another human Hox mutation causes synpolydactyly: extra fingers and toes fused together. A particularly devastating mutation that deletes no fewer than nine Hox genes in one go causes infants to be born with missing bones in the forearm, missing fingers and missing toes.

Limbs are not the only appendages in which Hox genes work. Infants born with Hox mutations that affect limbs tend to have malformed genitalia as well; in the worst cases male infants have just the vestiges of a scrotum and penis. Many of the molecules that make limbs also make genitals, and it should be no surprise that some mutations afflict both. The widely
rumoured positive correlation between foot and penis size also, surprisingly, turns out to be at least partly true. No man should be judged by the size of his feet, however, for the correlation, though statistically significant, is weak. And then, such data as there are concern ‘stretched’ rather than erect penis length, surely the variable of interest. Still, when the French refer to the penis as
le troisième jambe, pied de roi
or
petit-doigt
; and the English to the
best-leg-of-three, down-leg
or
middle-leg
, not forgetting the optimistic
yard
which elsewhere means three feet, they speak truer than they know.

The Hox genes have also begun to tell us about origins. Where do fingers come from? It may seem that this question has a straightforward answer. Our limbs, flexible in so many dimensions, are the cognates of the structures that propel fish through the sea: their fins. But fish don’t have fingers. One might suppose that the rays, those fine, bony projections that spread a fin like a fan, are their piscine equivalents. But fish rays and tetrapod digits are made of quite different kinds of bone – reason enough, anatomists say, to conclude that they have nothing to do with each other.

Most fish are only distantly related to tetrapods, so perhaps their want of fingers is no surprise. But even our closest piscine relatives are not much help. These are the lobe-finned fishes, among them the Australian lungfish, which spends much of its time buried in desiccated mud-flats, and the coelacanth, which inhabits the deeps of the Indian Ocean. Today’s lobe-fins are often called ‘living fossils’, an allusion to the abundance of their
relations four hundred million years ago and their scarcity now. Some fossil lobe-fins have fins that are strikingly like our own limbs; they seem to have cognates of a humerus, radius and ulna. They also have an abundance of smaller bones that look a bit like digits and that are made of the right kind of bone. But the geometry of these little bones is quite different to the stereotyped set of fingers and toes that is the birthright of all tetrapods. One can twist and turn a lung-fish’s fin as much as one pleases, but the rudiments of our hands and feet simply do not appear. The conclusion seems unavoidable: fish don’t have fingers, tetrapods do, and somewhere, around 370 million years ago, something new was made.

But how? Fish fin-buds are a lot like tetrapod limb-buds. They have apical ectodermal ridges, fibroblast growth factors, zones of polarising activity, sonic hedgehog, and panoplies of Hox genes that switch on and off in complicated ways as the bud pushes out into space. This tells us (what we already knew) that fins, legs and wings, so various in form and function, evolved from some
Ür
-appendage that stuck out from the side of some long-extinct
Ür
-fish.

We, however, are interested in the differences. One such difference lies in the details of the Hox genes. Early in the development of either a fin or a limb, Hoxd13 is switched on in the tailmost half, just around the zone of polarising activity. But as fins and limbs grow, differences begin to appear. In fish, the reign of Hoxd13 is brief; as the fin-buds grow it just gradually fades away. In mice, however, Hoxd13 stays on in an arc that stretches right across the outermost part of the limb. It seems
to be doing something new, something that is not, and never has been, done in fish: Hoxd13 is specifying digits.

Such differences (which are true of other Hox genes as well) give Hox gene mutations their deeper meaning. If, in its last flourish of activity, Hoxd13 is specifying digits, one would expect that a mouse in which Hoxd13 has been deleted would be a mouse with no digits. It would be a mouse in which just one of the many layers of change that have accreted over the course of five-hundred-odd million years of evolution has been stripped away. Its paws would be atavistic: incrementally less tetrapod-like and incrementally more fish-like. As it turns out, however, Hoxd13-mutant mice, far from having a lack of digits, have a surplus of them. Their digits are small and crippled, but instead of the usual five, they also have a sixth.

This result is rather puzzling. It seems to suggest that something, somewhere, in our evolutionary history not only had fingers and toes, but had more of them than we, and nearly all living tetrapods, do. The idea that Polydactyly (be it in mice, guinea pigs, dogs, cats or humans) is an atavism is an old one. Darwin claimed as much in the first edition of his
The variation of animals and plants under domestication
(1868), a work in which he attempted to develop the theory of inheritance that evolution by natural selection so badly needed. ‘When the child resembles either grandparent more closely than its immediate parent,’ he wrote, ‘our attention is not much arrested, though in truth the fact is highly remarkable; but when the child resembles some remote ancestor or some distant member of a collateral line, – and in the last case we must attribute this to the descent of all
members from a common progenitor, – we feel a just degree of astonishment.’

This is certainly true, but Darwin’s reasons for thinking that Polydactyly in humans is an atavism (or ‘reversion’ to use his terminology) are, to say the least, obscure. Salamanders, he noted, could regrow digits following amputation, and he had read somewhere that supernumerary fingers in humans could do the same thing even if normal ones could not. Extra digits were somehow, then, the product of a primitive regenerative ability, and hence atavisms.

It was a woolly argument, and it did not go unchallenged. The German anatomist Carl Gegenbauer pointed out that human fingers, supernumerary or otherwise, could not regenerate if amputated, and even if they could, so what? Polydactyly could not be an atavism without a polydactylous ancestor, and all known tetrapods, living or dead, had five fingers. In the next edition of
The variation
seven years later, Darwin, ever reasonable, admitted that he’d been wrong: polydactylous fingers weren’t atavisms; they were just monstrous.

But Darwin may have been right after all – albeit for the wrong reasons. In the last ten years or so, the ancestry of the tetrapods has undergone a radical revision. New fossils have come out of the rocks, and strange things are being seen. Contrary to all expectations, humans – and all living tetrapods –
do
have polydactylous ancestors. The earliest unambiguous tetrapods in the fossil record are a trio of Devonian swamp-beasts that lived about 360 million years ago:
Acanthostega, Turlepreton
and
Ichthyostega
. All of them are, by modern
tetrapod standards, weirdly polydactylous:
Acanthostega
has eight digits on each paw,
Turlepreton
and
Ichthyostega
have either six or seven. Suddenly it seems quite possible that Hoxd13-mutant mice, and mutant polydactylous mammals of all sorts, are indeed remembrances of times past – only the memory is of an early amphibian and not a fish.

Perhaps more genetic fiddling is required to get back to a fish fin; more layers have to be removed. This seems to be so. Mice that are mutant for Hoxd13 may be polydactylous, but mice that are mutant for Hoxd13 as well as other Hox genes – that is, are doubly or even trebly mutant – have no digits at all. It may be that as developmental geneticists strip successive Hox genes from the genomes of their mice, they are reversing history in the laboratory; they are plumbing a five-hundred-million-year odyssey that reaches from fish with no fingers to Devonian amphibians with a surplus of them, and that ends, finally, with our familiar five.

[O
N SKELETONS
]